The present invention relates to a metal matrix composite, including a light metal alloy, e.g., aluminum alloy, magnesium alloy, etc., as a matrix (base metal), and a piston using the same.
Conventionally, steel materials have been used as materials for mechanical element components. However, light metal alloys, such as Al (aluminum) alloy, Mg (magnesium) alloy, etc., are used for components that require reduction in weight. For some of components that require high-temperature strength, as well as reduction in weight, moreover, a metal matrix composite (abbrev. as MMC) may be used in the case where required characteristics cannot be obtained with use of a simple light metal alloy with a low melting point (i.e., with low high-temperature strength) or if the required characteristics cannot be obtained with use of a simple light metal alloy with poor wear resistance. The metal matrix composite is composed of a metallic matrix and reinforcements. Carbon fibers or ceramic fibers, such as SiC (silicon carbide), Al2O3 (alumina), etc., are used for the reinforcements, for example.
Components such as automotive parts and aircraft parts of which the weight is closely associated with the fuel-efficiency eagerly require reduction in weight. Materials for the components of this type are being changed from the conventional steel over to light metal alloys, such as Al alloy, Mg alloy, etc. To meet this requirement, materials for internal-combustion engines that exposed to high temperature and their peripheral parts (engine parts such as pistons, cylinder heads, cylinder blocks, connecting rods, etc.) are being changed over to light metal alloys. With the progress of development of higher-output internal-combustion engines, however, high-temperature strength and wear resistance have ceased to be ensured with use of a simple light metal alloy with a low melting point (i.e., with low high-temperature strength) or a simple light metal alloy with poor wear resistance. The following is a description of a piston for a diesel engine of an automobile as an example.
Direct-injection engines have recently been becoming prevailing. The load on the side of the combustion chamber of the piston is expected to increase as the development of higher-output versions will advance hereafter. The combustion chamber for forming eddies of air called swirls is formed in an end face of the piston. Since the edge (lip portion) that requires machining for finishing is thin-walled, in particular, it is hard to secure satisfactory fatigue strength in a high-temperature zone (e.g., at 300xc2x0 C. or thereabout) with use of conventional aluminum alloys (AC8A, etc.) for castings. The following is a description of the chemical ingredients of AC8A. In this specification, the chemical ingredients of the alloys are given by % by weight unless otherwise specified.
Cu: 0.8 to 1.3
Si: 11.0 to 13.0
Mg: 0.7 to 1.3
Zn: 0.15 or less
Fe: 0.8 or less
Mn: 0.15 or less
Ni: 0.8 to 1.5
Ti: 0.20 or less
Pb: 0.05 or less
Sn: 0.05 or less
Cr: 0.10 or less
Al: Remainder
Composites that use these Al alloys as their base metal (matrices) may possibly be subjected to surface treatment to improve their high-temperature fatigue strength. Since the effect of the strength improvement by the surface treatment is small, however, a metal matrix composite (MMC) is expected to be used.
Feasible reinforcements for the metal matrix composite include metallic fibers, carbon fibers, and ceramic fibers, and besides, porous structures and whiskers (crystal whiskers) formed of these materials, etc. Under the present conditions, fibers that are used as the reinforcements of the metal matrix composite are ceramic fibers, such as SiC, Al2O3, etc., and metallic fibers have not reached the level of practical use yet. This is so because no manufacturing technique has been established yet for metallic fibers of fiber diameters (several micrometers to tens of micrometers) that are required of reinforcements of a metal matrix composite, so that low-cost metallic fibers to serve for practical use cannot be obtained.
With the recent advance of performance that is required of various apparatuses, in particular, there is a growing tendency for higher fatigue strength or higher wear resistance to be required. Metallic fibers that can meet this high-level requirement are very hard and fragile, though not harder or more fragile than ceramics, so that they cannot be manufactured by the conventional wire drawing.
Usually, the casting method is used to compound a matrix and reinforcements. In the casting method, a preform (preformed piece previously molded to have a given shape and volume content) of fibers that serve as reinforcements is set in a mold. Thereafter, a molten matrix metal is poured into the mold. The preform is compulsorily impregnated with the matrix metal under a given pressure. A metal matrix composite is obtained by hardening the matrix metal.
In the case where carbon fibers or ceramic fibers are used for the reinforcements, they involve the following problems.
Carbon fibers and ceramic fibers have poor wettability with a light metal alloy that forms a base metal (matrix). Therefore, the light metal alloy of the matrix, e.g., Al alloy, fails to get well into spaces between the fibers, so that a large number of cavities (voids) are created inevitably. These defects lower the initial strength of the metal matrix composite and worsen the durability against corrosion or the like.
In order to improve the wettability with the matrix metal, therefore, the surface quality of the reinforcements that are formed of carbon fibers or ceramic fibers may be improved by plating or the like. However, the improvement of the surface quality requires many processes and much time, thus resulting in an increase in cost. Metallic fibers have a great advantage over carbon fibers and ceramic fibers with respect to the wettability with the matrix metal. As mentioned before, however, metallic fibers that are suited for reinforcements are expensive. It is hard for fibers of relatively low-priced stainless steel (SUS) to fulfill the high-level requirement for the high-temperature fatigue strength, wear resistance, etc.
Moreover, a composite that uses carbon fibers or ceramic fibers as its reinforcements must be preformed in order to prevent deformation of the reinforcements during casting operation. Preforming the carbon fibers or ceramic fibers requires a binder for use as an adhesive agent, and this binder causes the performance of the metal matrix composite to worsen.
A mold pressing method, extrusion molding method, and centrifugal molding method are known methods for manufacturing a preform with use of a binder. Any of these method requires many processes including a process for loosening fibers, process for applying the binder, temporary molding process, drying process, sintering process, etc.
There is also a problem that the composite using carbon fibers or ceramic fibers cannot be machined with ease. Primarily, carbon fibers and ceramic fibers are unworkable materials, so that it is natural that the metal matrix composite that uses them as its reinforcements cannot be worked with ease. Accordingly, there is a problem that the composite using carbon fibers or ceramic fibers entails prolonged working time or requires an expensive cutting tool. It is to be desired also in consideration of these circumstances that metallic fibers should be used for the reinforcements.
A metal matrix composite in which metallic fibers or ceramic fibers for use as reinforcements are mixed in a matrix metal of Al alloy has been developed as means for reducing the weight of and enhancing the strength of an engine piston. The casting method is adopted as a method for manufacturing the metal matrix composite of this type. Normally, heat treatment is carried out to enhance the mechanical strength of the composite after casting operation. There are close relations between conditions for the heat treatment and the chemical ingredients of the matrix (Al alloy). The Japanese Industrial Standards (JIS.H5202) provide the heat treatment conditions for the Al alloy of this type.
The aforesaid heat treatment includes a solution-treatment process for solidly solving additive elements in the alloy at high temperature and an age hardening process for extracting again the additive elements that are conducive to the improvement of the mechanical strength of the alloy after the solution-treatment process. According to a study made by the inventors hereof, however, it was recognized that the properties (e.g., fatigue strength) of the metal matrix composite worsen if the aforesaid heat treatment provided by JIS.H5202 is executed for the metal matrix composite in which the reinforcements of metallic fibers are mixed.
The aforesaid AC8A material that is a typical Al alloy for casting, for example, is loaded with Si, Ni, Mg, Cu. etc. as additive elements in order to restrain the coefficient of thermal expansion and improve the mechanical strength. According to JIS, heat treatment conditions for the AC8A material include 510xc2x0 C. and 4 hours for the solution-treatment process and 170xc2x0 C. and is 10 hours for the age hardening process. Hereinafter, this heat treatment will be referred to as T6 treatment. The following problems were aroused when the T6 process was applied to a metal matrix composite.
Let it be supposed, for example, that reinforcements, formed of FeCr metallic fibers of stainless steel or the like, are compounded with an Al alloy (matrix) and subjected to the T6 treatment. In this case, reactions occur on the interfaces between the matrix and the reinforcements, and intermetallic compounds such as FeAl, FeAl3, etc. are formed. Although these intermetallic compounds are very hard, they are fragile, so that the fatigue strength of the composite is adversely affected.
On the other hand, the heat treatment is supposed to be executed after ceramic fibers of B2Al2O6 or the like, for use as reinforcements, are compounded. In this case, reactions also occur on the interfaces between the matrix and the reinforcements, and an oxide compound such as MgAl2O4 is formed. Since this oxide compound, like the intermetallic compounds, is very fragile, so that the fatigue strength or the like of the composite is adversely affected.
The amount of formation of the intermetallic compounds of the composite that uses the metallic fibers is much greater than the amount of formation of the oxide compound obtained when the ceramic fibers are used, and the level of the bad influence is higher.
A method for coating the surfaces of the fibers used in the reinforcements with a film that cannot easily react with the matrix, e.g., an Al2O3 film that is chemically stable, can be adopted as means for solving the above problems. However, this method is not preferable because of its high cost.
Accordingly, the object of the present invention is to provide a metal matrix composite, using metallic fibers for reinforcements and enjoying excellent strength, wear resistance, etc. without subjecting the reinforcements to any surface treatment such as coating, and a piston using the same.
In order to achieve the above object, according to the present invention, there is provided a metal matrix composite having a metallic matrix and reinforcements mixed in the matrix, in which the reinforcements are formed of an alloy consisting mainly of Fe and Cr and containing Al and/or Si. The Cr content and the Al and/or Si content of metallic fibers that meet the object of the present invention range from 5 to 30% and from 3 to 10%, respectively.
Fe (iron), Ni (nickel), or Ti (titanium) may possibly be used as a metal to serve as the base of an alloy that constitutes the reinforcements (metallic fibers) of the present invention. Since Ni and Ti are too expensive to be adopted, however, Fe is used as the base metal. The oxidation resistance is improved by adding Cr. Normally, compounding a matrix metal and reinforcements requires preheating of preformings of the reinforcements.
The preheating is carried out in order to improve the wettability with the matrix metal first. The higher the temperature, in general, the better the wettability of reinforcements with a matrix is. In the process of cooling the matrix metal and the reinforcements after they are joined together, defects are liable to be caused if the difference in shrinkage between them is great, so that preheating is required. In the case of ceramic fibers, preheating to 600xc2x0 C. to 800xc2x0 C. is necessary in order to prevent occurrence of defects, even if the surface quality is improved by plating or the like.
On the other hand, metallic fibers is higher in wettability with the matrix metal than ceramic fibers, and their coefficient of thermal expansion is relatively close to that of the matrix metal. Accordingly, the metallic fibers have an advantage over the ceramic fibers in being satisfactorily preheated to a lower temperature (500xc2x0 C. or below) than the ceramic fibers is. However, the metallic fibers have a problem of oxidation by preheating.
For example, an oxide film may possibly be formed on the surface of each metallic fiber during the preheating process. If this oxide film is an oxide of Fe (Fe2O3), the wettability with the matrix metal is poor. Thus, the matrix metal cannot easily get into spaces between the fibers. Since the oxide film easily separates from each metallic fiber, moreover, defects are caused. In order to improve the oxidation resistance during the preheating process, according to the present invention, therefore, oxidation of the base metal (Fe) of the reinforcements is prevented by adding Cr.
The inventors hereof manufactured a plurality of types of test pieces with varied quantities of Cr added to the base metal (Fe), by arc solution-treatment, and conducted an oxidation test. According to the method of the oxidation test, the manufactured test pieces were left to stand for two hours in the atmosphere in electric ovens at different ambient temperatures. After these test pieces were taken out of the electric ovens, the colors of the respective surfaces of the test pieces were visually observed and further observed by means of an electron microscope (SEM), and the surface conditions were checked to see if they were changed by heating. By an analysis by means of an EDX (energy dispersed X-ray spectrometer), moreover, the presence of oxides in the test pieces was examined.
The results of the above examinations are shown in TABLE 1. Based on these examinations, it was confirmed that an oxidation preventing effect is produced with the Cr content at 5% or more. A very small quantity of oxygen was detected in the EDX analysis with the Cr content at 5%. However, this is negligible because it is a dense, very thin Cr oxide that has good wettability with the matrix metal and good adhesion to the metallic fibers. Preferably, therefore, the Cr content should be 5% or more. In order to improve the safety (reliability) further, however, the Cr content should preferably be 10% or more.
Performance that meets high-level requirements for the high-temperature fatigue strength and wear resistance cannot be obtained by only adjusting the Cr content to the aforesaid value (5% or more). According to the present invention, therefore, Al and/or Si that is low-priced is added as an element for improving the strength, hardness, and thermal resistance. Test pieces for which the quantity of Al or Si added to FeCr for use as a base metal is changed variously were manufactured by arc solution-treatment, and the oxidation resistance was evaluated by carrying out the same oxidation test as aforesaid. At the same time, a tensile test on the test pieces was conducted at the ambient temperature of 300xc2x0 C. The ambient temperature of 300xc2x0 C. is equivalent to the working atmosphere temperature of internal-combustion engine parts (e.g., pistons, etc.). Further, the degree of difficulty of fiberization based on drawing of the alloy material and the degree of difficulty of fiberization by the melt extraction method were examined.
TABLE 2 shows the results of the above tests for the case where Si was added. The same tendency was observed for the case where Al was added. In TABLE 2, ◯, xcex94, and x represent good, passable, and failure, respectively.
As seen from TABLE 2, metallic fibers that can display higher strength (800 MPa or more) than stainless steel and can be fiberized preferably have Cr contents of 5 to 30% and Al and/or Si contents of 3 to 10%.
TABLE 3 shows the diameters and cuttability of metallic fibers that can be manufactured by the melt extraction method. Preferably, as shown in TABLE 3, the lower limit value of the fiber diameter should not be lower than xcfx8620 xcexcm, which is the lower limit for the fiberization by the melt extraction method. As for the upper limit value of the fiber diameter, it is expected to be not higher than xcfx86100 xcexcm in consideration of the post-workability (cuttability) after compounding. Symbol xcfx861 represents the diameter of each fiber. The cross section of each reinforcement (metallic fiber) may be perfectly circular. Preferably, however, the cross section should be irregular in the circumferential direction, like those of the metallic fibers that are manufactured by the melt extraction method, since the bite (anchor effect) on the matrix is then improved. By the melt extraction method, moreover, even metallic fibers that are formed of an unworkable material can be manufactured at relatively low costs.
According to the present invention, there may be obtained a metal matrix composite that enjoys outstanding fatigue strength and wear resistance at high temperature, in particular. Since the metal matrix composite of the present invention is dissolved at 470xc2x0 C. to 500xc2x0 C., reactants such as intermetallic compounds can be restrained from being formed on the interfaces between the matrix and the reinforcements, and the fatigue strength can be improved further.
Since the metal matrix composite of the present invention is substantially formed of metals only, it can be recycled with ease. Since the respective mechanical properties of the matrix and the reinforcements are relatively similar, moreover, cutting that is carried out after casting is easy, and the working time and working cost can be reduced considerably. Since the reinforcements (metallic fibers) can be preformed without using any binder, furthermore, an application process, temporary molding process, drying process, etc. for a binder can be omitted. A piston that uses the metal matrix composite of the present invention for a lip portion of its combustion chamber has excellent machinability and enhanced high-temperature strength, and can be recycled.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.